World Rugby refused to comment directly on Leo's allegations but reiterated its commitment to "ensure the integrity" of the international game remains intact.

Leo, who claims to be speaking from experience, says the players who choose to represent Samoa, Fiji or Tonga at the World Cup face losing nearly half of their salary.

Leo, who is leaving Irish at the end of the season, said: "There's no hints or beating around the bush, they come out and say 'stop playing for your country'. It is standard practice. I have not made a decision about whether I will be at the World Cup because I am struggling to find a club that will pay me during the tournament. If I want to play professionally next year, I may have to forfeit ­playing at the World Cup."

Leo said that Census Johnston, the tight-head prop, recently signed a new contract with Toulouse on the proviso that he retired from international duty with Samoa.

"Almost every Pacific Island player I have spoken to has had pressure put on them to retire [from international rugby]," Leo said. "If everyone could speak as openly as I do, they would say the same thing. Everyone is getting those pressures put on them. When they are negotiating a deal with the clubs, they will say 'we will give you this bit more if you don't play for your country'.

"Census is our most experienced player and by far our best tight-head. So if you take out a player like him, Paul Williams and myself, who are three guys coming out of contract, that's a massive leadership drain on the squad. How do you replace that?

"That's a very real thing for our management to deal with, hence they have named a 58-man training squad for the World Cup. Out of those 58, 12 or 13 will have their contracts coming up. If they decide to stay at their clubs then we lose a third of our [World Cup] squad."

World Rugby's Regulation Nine states that: "The future development and extension of the sport at all levels and throughout the world would be threatened if a Union was not able to select and have available the players it requires."

Clubs would argue that they have done nothing wrong as retiring players are making an individual decision, but it would appear to contradict World Rugby's demand of "compliance with not only the letter of the regulation, but also its spirit".

A World Rugby spokesman said: "While it would be inappropriate to comment on any allegations without possession of the facts, World Rugby works tirelessly with its unions to ensure that the spirit of regulation nine and the integrity of the international game is preserved."